The principal investigator is interested in the mechanisms by which human pathogenic viruses replicate and establish chronic infections. For the past five years, such interests have been directed toward studies of the replication of hepatitis delta virus (HDV). This subviral human pathogen establishes acute and chronic infections of the liver and uses a helper, hepatitis B virus (HBV), to provide envelope proteins needed for virus assembly. The 25 million carriers worldwide who are chronically infected with HDV suffer a greater incidence of active hepatitis, liver cirrhosis and hepatocellular carcinoma than do those infected with the helper virus alone. The mechanisms responsible for such HDV-associated pathogenicity remain poorly understood and there are currently no effective treatments for HDV-infected individuals. A more detailed understanding of the molecular details involved in HDV replication will be essential if such antiviral therapies are to be developed. In addition to its clinical importance, HDV is also of scientific significance. Unlike all other infectious agents of animals, HDV contains a circular, single-stranded RNA genome that encodes ribozymes. These ribozymes self-cleave multimeric replication intermediates into unit-length species and the resulting termini are joined in a ligation reaction to generate the monomeric circular species. HDV also expresses a protein, the delta antigen, that binds viral RNA to form a ribonucleoprotein (RNP) complex. Like the RNA processing reactions, both the assembly of this RNP and its subsequent incorporation into HBV envelope particles represent essential steps in the viral life cycle that are logical targets for antiviral intervention. Both biochemical and genetic methods will be used in an effort to otter understand the process by which the delta antigen specifically identifies and assembles on its target RNA. Additional experiments are proposed to determine the domain within the helper virus envelope protein responsible for RNP packaging. This will be accomplished by making hybrids with a relative of HBV, duck hepatitis B virus, which is unable to package the HDV RNP. Such experiments may also enable the development of a new animal model useful for the study of HDV pathogenesis. Finally, the principal investigator has previously established a role for host-specific factors in the processing of HDV RNA. To better understand their role in both HDV and host RNA maturation, these factors will be cloned and biochemically characterized.